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Article

Production of a Novel Biomedical β-Type Titanium Alloy Ti-23.6Nb-5.1Mo-6.7Zr with Low Young’s Modulus

by
Aline Raquel Vieira Nunes
1,*,
Sinara Borborema
2,
Leonardo Sales Araújo
1,
Luiz Henrique de Almeida
1 and
Michael J. Kaufman
3
1
Department of Metallurgical and Materials Engineering, Federal University of Rio de Janeiro, Rio de Janeiro 21941-630, RJ, Brazil
2
Department of Mechanical and Energy, Rio de Janeiro State University, Resende 27537-000, RJ, Brazil
3
Center for Advanced Non-Ferrous Structural Alloys, Colorado School of Mines, Golden, CO 80401, USA
*
Author to whom correspondence should be addressed.
Metals 2022, 12(10), 1588; https://doi.org/10.3390/met12101588
Submission received: 10 August 2022 / Revised: 13 September 2022 / Accepted: 20 September 2022 / Published: 24 September 2022
(This article belongs to the Special Issue Innovations in Metallic Biomaterials)
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Abstract

:
Metastable β titanium alloys are developed for biomedical applications due to their low Young’s moduli and functional properties. These alloys can be fitted to different parts of orthopedic implants through thermomechanical processing and chemical composition control. This study aimed to produce, process, and characterize a new metastable β titanium Ti-23.6Nb-5.1Mo-6.7Zr alloy on a semi-industrial scale for orthopedic implant manufacturing, and to discuss the influence of the cold rolling and transformed phases during aging in the microstructure and mechanical properties. This alloy was produced in a vacuum arc remelting furnace (VAR) and thermomechanically processed under different conditions. The samples were characterized by X-ray diffractometry, optical, and scanning electron microscopy, and Young’s modulus (YM) and Vickers Hardness (HV) tests. Among other processing conditions, the sample that was 50% cold rolled after solution treatment, which resulted in a microstructure with β and α″ phases, had the lowest YM (~57 GPa), and the sample aged at 300 °C for 2 h had the highest HV/YM ratio (5.42). The new alloy produced in this work, processed by different routes, showed better mechanical properties than most recently developed metastable Ti-β Alloys.

Graphical Abstract

1. Introduction

β-type implants are the best-suited titanium alloys for orthopedic implants due to their biocompatibility, superior corrosion resistance in the body environment, and high strength combined with low YM [1,2]. These alloys consist of titanium combined with biocompatible β-type elements, such as Mo, Nb, Sn, and Ta [3]. Special attention has been paid to Zr as an alloying element that ultimately stabilizes the beta phase and alters the precipitation conditions of the α″ phase during the conformation and ω aging processes [4].
Alloys with a β phase exhibit a lower YM compared with those with α and α + β phases. β-phase alloys also comply with most requirements for metallic implants [5,6]. In recent years, new β-type alloys have been developed for biomedical applications with low YM [7,8]. However, obtaining titanium alloys with a low YM and high mechanical strength remains challenging [9].
For titanium alloys, the Young’s modulus results from the balance between the elastic moduli of each phase, as well as the crystal orientation and the aspect of precipitates [10]. Thus, this property can be manipulated through the addition of alloying elements and thermomechanical treatments for the stabilization of different phases [11,12]. For alloys with higher percentages of alloying elements, the interaction forces between atoms tend to be higher, often exhibiting an improved strength and a higher Young’s modulus [13].
Independent studies confirm that the ω phase presents the highest YM among all phases of titanium alloys [4,14]. However, the literature contains controversial findings regarding the β and α″ phases, as the latter can exhibit different Young’s moduli when hardening by precipitation or when deformation-induced [5]. According to Zhou and Niinomi [15], the β phase presented the lowest YM, followed by the α″ and α phases. Nevertheless, Zhou et al. [16] showed that the α″ and β phases presented similar Young’s moduli for the Ti-30Ta and Ti-70Ta alloys. However, Hanada et al., [17,18] in their studies with β Ti-33.6Nb-4Sn titanium alloys, obtained a YM of 45 GPa through the development of a stress-induced α″ phase after several forging steps.
In β-type alloys with a large number of β-stabilizer elements, ω precipitation is observed during aging. This precipitation is observed at different aging temperatures, depending on the alloy composition, though it usually takes place between 100 °C and 500 °C. This behavior was first described by Laheurte et al. [19] using different temperatures and aging times based on TTT diagrams for the β-III Ti-11.5Mo-6Zr-4.5Sn alloy. The authors observed ω phase precipitation during aging from 250 °C to 500 °C. At higher aging temperatures, precipitation of the α phase without a second ω phase was also observed [19].
Generally, ω precipitation can be reduced by adding Zr [20] and Sn [21]. For alloys of the Ti-Nb system with 13% to 18% Nb, small quantities of the ω phase can be formed by cold rolling. This phase is also present in alloys with Nb content ranging from 9% to 30% after aging at 450 °C. At this temperature, the β matrix dissolves the Nb until the solubility limit, thereby reaching a metastable balance between the β and ω phases.
Niinomi et al. [22] developed the Ti-29Nb-13Ta-4.6Zr alloy, which, after solution treatment, exhibits a YM of 63 GPa. Samples of this alloy were solubilized at 790 °C and aged at 300 °C to 400 °C for 72 h with ω phase precipitation. For samples aged at 400 °C, acicular α phase precipitates were also present. Young’s moduli tended to vary according to which thermomechanical treatment was applied. The lowest YM was obtained for the alloy subjected to water quenching after solution treatment and severe cold working (~55 GPa). In comparison, the highest YM was achieved by aging after solution treatment (~97 GPa).
This study aimed to characterize a newly engineered β titanium alloy, Ti-23.6Nb-5.1Mo-6.7Zr, produced on a semi-industrial scale to manufacture orthopedic implants. The effects of thermomechanical processing on the final microstructure and mechanical properties were analyzed. A single-phase β structure, after solution treatment and followed by water quenching, was obtained. The α″ martensite was induced after 50% deformation by cold rolling. The α and ω phases were precipitated after aging at higher temperatures. The effect of these phases on hardness and YM was discussed.

2. Materials and Methods

A cylindrical 6.5 kg ingot measuring 120 mm in diameter by 125 mm in height was cast in a vacuum arc remelting furnace (VAR). A titanium bar (grade 2-ASTM B348) 50.8 mm in diameter by 850 mm in height served as the electrode added to high purity Zr, Mo, and Nb in a quantity by a weight proportional to the intended chemical composition. For the aggregation of the alloying elements, 57 holes were machined on the electrode surface, each measuring 12.7 mm in diameter and 45 mm in depth, to obtain a more even distribution of the elements during melting. To avoid segregations and ensure the homogeneity of the alloy produced, the ingot was remelted three times. Figure 1 shows the electrode before melting and the final produced ingot.
The chemical composition of the alloy was based on the phase stability diagram proposed by Abdel-Hady, related to the Bo/Md parameters [11] and on the molybdenum equivalence ([wt%Mo]eq). The Bo parameter measures the strength of the bond between titanium and the other atoms. Md is the average orbital energy level of the element in the alloy and correlates with the electronegativity and the metallic radius. The minimum value required to stabilize the β phase upon water quenching is a Moeq of 10 wt% [23]. The nominal composition was determined by X-ray fluorescence spectrometry, and gas analysis (interstitial oxygen and nitrogen contents) was carried out by inert gas extraction in a gas analyzer.
After remelting, the ingot was machined in to a rectangular shape with dimensions of 80.5 mm × 14 mm × 13 mm. Solution treatment was carried out at 1000 °C for 24 h, after which the ingot was quenched in cold saline. After solution treatment, the ingot was cold rolled at 50% reduction with successive rolling steps of 0.5 mm until it reached the desired width reduction without intermediary heat treatments. The degree of cold rolling reduction was based on previous works conducted by Nunes et al. [24,25]. After cold rolling, encapsulated samples under an argon atmosphere were aged at 300 °C, 400 °C, and 500 °C for 2 h, then water quenched. Aging treatment conditions were adopted according to the works carried out by Hanada et al. [17], and by Gabriel et al. [26]. Cold-rolled, encapsulated samples under vacuum pressure were also annealed at 950 °C for 1 h and subsequently water quenched at room temperature.
Phase identification was carried out using an X-ray diffractometer operated at 40 kV and 30 mA with Ni-filtered CuKα radiation. Microstructural characterization was performed by optical microscopy (OM). The samples were ground with sandpaper from 100 to 4000 mesh, polished in 90% colloidal silica—10% distilled water solution for 30 min, and then etched with Kroll’s reagent (8 mL HF, 20 mL HNO3, and 62 mL H2O).
Vickers Hardness (HV) values were obtained with a load of 200 gf for 15 s. Young’s modulus was determined using the impulse excitation technique according to ASTM E 1876. YM of a standard Ti-6Al-4V alloy was also calculated under the same conditions for comparison purposes. Ten measurements were taken for each condition in both tests. It is important to remark that the hardness presents a well-established linear dependence to the strength, as described in the literature [27], and this relationship was used to calculate the HV/E ratio, as shown in the results section.

3. Results and Discussion

3.1. Microstructural Characterization

Table 1 shows the chemical composition of the alloy, which had a Moeq of 11.6 wt%. Oxygen and nitrogen concentrations were below the maximum values established for titanium grade 2, according to ASTM guidelines F67-06-06, at 0.25% and 0.03% (% in mass), respectively.
Figure 2 shows X-ray diffraction (XRD) patterns and micrographs of the sample as cast and after being solution-treated (ST) at 1000 °C for 24 h and then water quenched. The XRD patterns in Figure 2a indicate the presence of the β phase only. Similar findings were described by Mythili et al. [23] and Ho et al. [28], who from 10 wt% [Moeq] obtained single β-phase alloys, as the alloy sample exhibited 11.6 wt% [Moeq]. The micrograph in Figure 2b highlights the presence of dendrite structures without any apparent segregation characteristics of casting metals. The ST sample (Figure 2c) exhibited completely recrystallized coarse equiaxed grains in the β phase, ranging from 856 μm to 1678 μm in size. These data indicate that the treatment eliminated heterogeneities typical of melting processes.
Figure 3 presents the XRD patterns and micrographs of the 50% cold-rolled (CR) sample. Figure 3a indicates the presence of β phase and α″ martensite induced by plastic deformation. The intensity of the β (110) phase decreased as a function of the reduction ratio during cold rolling [29]. The increase in the intensity of the β/α″ (200) phases corroborate the presence of stress-induced martensite transformations. The same effect was observed by Wang et al. [30] for the Ti-35Nb-2Ta-3Zr alloy obtained at a reduction rate of 99%, with the stress-induced α″ martensite being verified from 20% cold working.
The 50% CR sample (Figure 3b) shows the presence of coarse grains and β deformation twins within the grains. The twinning volume fraction in the grains varies as a function of grain orientation, with a greater volume observed for higher deformations [29,31,32]. In many β-type titanium alloys, high ductility results from mechanical twinning [33]. Deformation mechanisms were studied by Nunes et al. [34] for the Ti-29Nb-2Mo-6Zr alloy with 30% deformation by EBSD, having verified the presence of {332} <113> primary and {112} <111> secondary twins, corroborating the findings described in the literature.
The 50% CR samples were aged for 2 h at 300 °C, 400 °C, and 500 °C. Figure 4 shows the X-ray diffraction (XRD) patterns of the samples subjected to these aging conditions. The sample aged at 300 °C for 2 h revealed the presence of the α″ phase in the β matrix, which was also observed in the CR condition, suggesting that this combination of temperature and aging time was not sufficient to reverse this transformation and precipitate other phases. The sample aged at 400 °C for 2 h exhibited precipitation of the α and ω phases in the β matrix, with the presence of low-intensity peaks. Regarding the sample aged at 500 °C for 2 h, an increase in peak intensity of the β and α phases was verified along with the dissolution of the ω phase.
The microstructure evolution under aging at 300 °C, 400 °C, and 500 °C for 2 h after 50% CR is presented in Figure 5. For the sample aged at 300 °C for 2 h, Figure 5a,b show the presence of the needle-like precipitation of the α″ martensite phase and deformation twinning within coarse β grains. The morphology is similar to the microstructure of the CR sample. For the sample aged at 400 °C for 2 h (Figure 5c), coarse β grains were observed, and deformation twinning was verified within the grains. The presence of the α″ phase is no longer evident at this temperature, while the ω phase could only be identified by transmission electron microscopy due to its nanometric size. Regarding the sample aged at 500 °C for 2 h (Figure 5d), changes in its microstructure were observed due to the aging treatment, revealing the presence of the α phase in the β matrix.
Microstructures with morphologies similar to that observed in Figure 5d were also verified by Xu et al. [35] after cold rolling and aging the Ti-15Mo-3Al-2.7Nb-0.2Si alloy. They showed that defects formed during cold rolling provide enough energy for phase transformation during aging, with these defects favoring a finer α phase precipitation.
Figure 6 presents the X-ray diffraction (XRD) patterns and the micrograph for the sample annealed at 950 °C for 1 h followed by water quenching after 50% CR. In the XRD pattern of Figure 6a, only the presence of the β phase can be observed, which corroborates the metallographic analysis presented in Figure 6b of the complete recrystallization of the microstructure and grains approximately 50.4 µm in size. The reflection of the β phase, at approximately 70°, reveals β (211) texture. Cold rolling before annealing led to the formation of this preferential orientation; that is, to the appearance of the recrystallization texture. This texture has been observed in several studies on titanium alloys, such as the Ti-5Ta-1.8Nb alloy annealed at 1000 °C for 2 h after 83% cold rolling [36] and the Ti-24Nb alloy annealed at 900 °C for 1 h after 95% CR [37]. In these studies, the recrystallization texture of the previously deformed alloy is similar to the rolling texture developed during cold rolling by the α″ and β before annealing treatment.

3.2. Mechanical Properties of the Processed Alloy

Table 2 outlines the mechanical properties of all of the conditions present in the microstructural characterization. The table also presents the heat treatment conditions and the phases identified by XRD.
The recrystallization process led to a considerable reduction in hardness and mechanical strength due to the decrease in dislocation density and YM due to the development of recrystallization texture. Before the annealing treatment, with the 50% CR sample, the hardness and YM were ~271.80 HV and 57.62 GPa, respectively. However, after annealing at 950 °C for 1 h, a drop in the sample’s hardness (~246.90 HV) was observed due to the thermally-activated recovery and recrystallization processes. Nonetheless, YM remained unaffected (58.59 GPa).
The sample aged at 300 °C showed an increase in hardness and YM, although the presence of the α″ phase was observed. For the sample aged at 400 °C, hardness and YM were even higher due to the hardening microstructure associated with the presence of the nanometer-scale ω phase particles formed during aging. The presence of small ω phase peaks was only observed at 400 °C. The sample aged at 500 °C presented a reduction in hardness and YM compared with the sample aged at 400 °C; this decrease was observed due to recovery mechanisms and the suppression of the ω phase.
As previously described in the microstructural characterization section, the deformation mechanisms after the cold rolling were twinning and stress-induced α″ martensite. Likewise, Xu et al. [38] observed that the cold rolling of Ti-25Nb-10Ta-1Zr-0.2Fe alloy induced the α″ martensitic transformation inducing a decrease in YM. On the other hand, the fine-grained microstructure and the higher dislocation density increased the hardness [38]. The aging at 300 °C for 2 h did not cause the reversal of the α″ phase, increasing only hardness, but maintaining a low value of YM (63.4 GPa). The YM values for all the processing conditions were suitable for biomedical application, as compared to those recently published [39].
According to Lee et al. [40], for the Ti-Nb system with up to 35% of Nb, the microhardness of the individual phases varies in the following order ω > α′ > α″ > β > α, while the bending modulus (Equivalent to the YM) varies in the following order ω > α > α′ > α″ > β. These observations corroborate the higher values observed and described in Table 2 for both hardness and YM for the sample aged at 400 °C where ω phase is present. The decrease in both hardness and YM for the sample aged at 500 °C can be associated with the dissolution of the ω phase.

4. Conclusions

Among the different treatment conditions studied for the new Ti-23.6Nb-5.1Mo-6.7Zr titanium alloy, the highest hardness/Young’s modulus ratio was obtained for the 50% cold-rolled sample aged at 300 °C (HV/E = 5.42). This sample contained a microstructure with β and α″ phases and exhibited a low YM (~63 GPa). The sample that was 50% cold rolled after solution treatment presented the lowest value of YM (~57 GPa). The new alloy developed in this work presented values of hardness and YM comparable to the most suitable alloys described in the scientific literature, under all conditions tested in this study.

Author Contributions

Conceptualization, A.R.V.N., S.B., L.S.A., L.H.d.A. and M.J.K.; formal analysis, L.S.A.; funding acquisition, S.B.; investigation, A.R.V.N.; methodology, A.R.V.N., S.B. and L.H.d.A.; resources, L.H.d.A. and M.J.K.; supervision, L.H.d.A. and M.J.K.; visualization, A.R.V.N.; writing—original draft, A.R.V.N.; writing—review and editing, L.H.d.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Brazilian agencies CNPq (grants #161842/2015-1 and #204566/2018-5) and FAPERJ (grant #E-26/202.654/2019).

Data Availability Statement

The data of this study are available from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Metals 12 01588 g001 550
Figure 1. (a) Grade 2 Ti electrode with alloying elements fixed into machined holes prepared for remelting in a VAR furnace; (b) final produced ingot, 6.5 kg, 120 mm in diameter by 125 mm in height.
Figure 1. (a) Grade 2 Ti electrode with alloying elements fixed into machined holes prepared for remelting in a VAR furnace; (b) final produced ingot, 6.5 kg, 120 mm in diameter by 125 mm in height.
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Metals 12 01588 g002 550
Figure 2. Microstructural characterization of Ti-23.6Nb-5.1Mo-6.7Zr alloy as cast and completely recrystallized after being solution-treated at 1000 °C for 24 h and water quenched; (a) XRD pattern, (b,c) OM images. Black dots are preparation artifacts.
Figure 2. Microstructural characterization of Ti-23.6Nb-5.1Mo-6.7Zr alloy as cast and completely recrystallized after being solution-treated at 1000 °C for 24 h and water quenched; (a) XRD pattern, (b,c) OM images. Black dots are preparation artifacts.
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Figure 3. Microstructural characterization of Ti-23.6Nb-5.1Mo-6.7Zr alloy after 50% cold rolled; (a) XRD pattern, (b,c) OM images. Rolling direction (RD).
Figure 3. Microstructural characterization of Ti-23.6Nb-5.1Mo-6.7Zr alloy after 50% cold rolled; (a) XRD pattern, (b,c) OM images. Rolling direction (RD).
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Figure 4. XRD pattern of Ti-23.6Nb-5.1Mo-6.7Zr alloy after 50% cold rolled and aged for 2 h at 300 °C, 400 °C, and 500 °C.
Figure 4. XRD pattern of Ti-23.6Nb-5.1Mo-6.7Zr alloy after 50% cold rolled and aged for 2 h at 300 °C, 400 °C, and 500 °C.
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Figure 5. OM microstructures of Ti-23.6Nb-5.1Mo-6.7Zr alloy after 50% cold rolled and aged for 2 h at (a) 300 °C with (b) detail of a stress-induced α″ martensite, (c) 400 °C, and (d) 500 °C. Rolling direction (RD).
Figure 5. OM microstructures of Ti-23.6Nb-5.1Mo-6.7Zr alloy after 50% cold rolled and aged for 2 h at (a) 300 °C with (b) detail of a stress-induced α″ martensite, (c) 400 °C, and (d) 500 °C. Rolling direction (RD).
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Figure 6. Microstructural characterization of Ti-23.6Nb-5.1Mo-6.7Zr alloy annealed at 950 °C for 1 h after 50% cold rolled followed by water quenching; (a) XRD pattern, and (b) OM image.
Figure 6. Microstructural characterization of Ti-23.6Nb-5.1Mo-6.7Zr alloy annealed at 950 °C for 1 h after 50% cold rolled followed by water quenching; (a) XRD pattern, and (b) OM image.
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Table
Table 1. Chemical composition for the final ingot after solution treatment (Moeq = 11.6 wt%).
Table 1. Chemical composition for the final ingot after solution treatment (Moeq = 11.6 wt%).
Composition (% Mass)
TiNbMoZrON
Balanced23.57 ± 0.135.07 ± 0.056.72 ± 0.090.1050.0098
Table
Table 2. Vickers Hardness (HV 0.2), Young’s modulus (YM), and the HV/E ratio of the ST and 50% CR Ti-23.6Nb-5.1Mo-6.7Zr alloy aged at 300 °C, 400 °C, and 500 °C for 2 h.
Table 2. Vickers Hardness (HV 0.2), Young’s modulus (YM), and the HV/E ratio of the ST and 50% CR Ti-23.6Nb-5.1Mo-6.7Zr alloy aged at 300 °C, 400 °C, and 500 °C for 2 h.
AlloyThermomechanical
Processing		Temp.
(°C)		TimeHardness
(HV-0,2)		YM
(GPa)		HV/EPhases
(DRX)
Ti-23.6Nb-5.1Mo-6.7ZrST100024 h228.90 ± 2.08NMNMβ
50% CR--271.80 ± 6.7057.62 ± 0.324.72β + ⍺″
Annealed
(After 50% CR)		9501 h246.90 ± 2.9258.59 ± 0.374.21β
50% CR and Aged3002 h343.70 ± 4.2263.40 ± 0.635.42β + ⍺″
50% CR and Aged4002 h395.00 ± 4.1178.57 ± 2.005.03β + ⍺ + ω
50% CR and Aged5002 h351.60 ± 3.6069.47 ± 1.145.06β + ⍺
Ti-6Al-4VAs Cast--337 ± 2.0120 ± 3.02.81⍺ + β
NM—Not measured, CR—Cold Rolled.
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Nunes, A.R.V.; Borborema, S.; Araújo, L.S.; de Almeida, L.H.; Kaufman, M.J. Production of a Novel Biomedical β-Type Titanium Alloy Ti-23.6Nb-5.1Mo-6.7Zr with Low Young’s Modulus. Metals 2022, 12, 1588. https://doi.org/10.3390/met12101588

AMA Style

Nunes ARV, Borborema S, Araújo LS, de Almeida LH, Kaufman MJ. Production of a Novel Biomedical β-Type Titanium Alloy Ti-23.6Nb-5.1Mo-6.7Zr with Low Young’s Modulus. Metals. 2022; 12(10):1588. https://doi.org/10.3390/met12101588

Chicago/Turabian Style

Nunes, Aline Raquel Vieira, Sinara Borborema, Leonardo Sales Araújo, Luiz Henrique de Almeida, and Michael J. Kaufman. 2022. "Production of a Novel Biomedical β-Type Titanium Alloy Ti-23.6Nb-5.1Mo-6.7Zr with Low Young’s Modulus" Metals 12, no. 10: 1588. https://doi.org/10.3390/met12101588

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